Most useful alkane conversion processes (such as dehydrogenation and dehydrocoupling) are endothermic and generate hydrogen. Typical heterogeneous catalysts require high temperatures and usually exhibit low selectivities. Trivalent iridium complexes of tridentate bis(phosphine) ligands (i.e., ‘pincer’ ligands) have shown unprecedented activity for selective alkane dehydrogenation catalysis. Such homogeneous catalysts operate at lower reaction temperatures and are typically run in open systems to drive the reaction by allowing release of hydrogen. Alternatively, use of a bulky alkene acceptor, such as t-butylethylene, can also be used to absorb the hydrogen product and help drive the alkane dehydrogenation reaction. These catalysts have also recently attracted a great deal of attention from their use in a tandem alkane dehydrogenation/metathesis scheme for upgrading low carbon number refinery waste streams to higher carbon number fuels. However, these catalysts suffer from: 1) bimolecular decomposition reactions under operation conditions; 2) product inhibition—high alkene concentrations prevent binding of alkane substrates; 3) isomerization of targeted terminal alkene products to less valuable internal alkenes; and, 4) difficulties with catalyst separation and recycle. Immobilization of organometallic moieties on active surfaces can, in principle, circumvent all the problems noted above and will also allow dehydrogenation of gaseous substrates such as ethane and propane.
Previous examples of immobilization included reactions of Rh(allyl)3 with silica. Metal allyl complexes are potentially excellent to form a family of supported catalysts, since the allyl ligand can easily go through p-s conversion, thus allowing a variety of other electron donating ligands bonded to the metal. The resulting Rh(allyl)2(O—) species and related moieties were characterized in detail and shown to be active for alkene hydrogenation catalysis. The stability of these rhodium catalysts however was limited by reduction at the metal center (to rhodium metal) and organic fragment transfer to the surface (i.e., to afford allyl-O—Si species). Corresponding studies of the potentially more stable iridium analog were hampered by lack of an efficient route to Ir(allyl)3.
The high-yield preparation of Ir(allyl)3 was previously shown (John et al., Organometallics 2001, 20, 296) wherein a rich ligand substitution chemistry was also demonstrated to provide Ir(allyl)3(L)n without reduction of the trivalent iridium center even with a strong acid ligand of carbon monoxide. Unfortunately, the Ir(allyl)3 and its derivatives were found unreactive with active silica surfaces or substrates.